Valves are frequently used in fluid handling devices, such as metering pumps. In such an environment, a relief valve is often provided to provide a fluid pressure relief function when fluid pressures exceed a maximum value. Further, an automatic air bleed valve separate from the relief valve is also typically provided to allow continuous bleed off of small amounts of air liberated from the working fluid which would otherwise accumulate and be trapped in the working chamber of the pump. Still further, a purge valve may be included to permit rapid purging of relatively large amounts of air from the working chamber.
In at least one type of pump design as disclosed in U.S. Pat. No. 5,860,793, the purge valve function is incorporated into the automatic air bleed valve. Specifically, the automatic air bleed valve comprises a ball disposed in a valve body and movable between a lower valve seat and an upper valve seat. A rod is disposed on the top of the ball and is movable therewith. During each discharge stroke of the pump, the ball and rod move upwardly away from the lower seat toward the upper seat. During travel of the ball, a small amount of air and working fluid escape out of the working chamber. Once the ball moves into engagement with the upper seat, further escape of air and working fluid is prevented. On the subsequent suction stroke, the ball moves downwardly back into sealing engagement with the lower seat. The purge function is effected by depressing a pushbutton on the valve, thereby limiting the upward movement of the rod and preventing the ball from reaching the upper seat during the upward travel thereof. Because the ball is not sealed, a larger quantity of air and working fluid can escape from the working chamber. While a purge function can be undertaken in this fashion, the amounts of air that be purged are limited owing to the small clearances between the ball and the valve body. In addition, the pump disclosed in the '793 patent requires the relief valve in addition to the valve that accomplishes the automatic bleed and purge functions.